The carboxylic acid group of a rosin acid can be converted to an ester through a reaction with various alcohols. The alcohols typically used to make rosin esters are shown in Figure 1. The molecular weight and functionality of the alcohol determines the softening point of the subsequent ester. Glycerol with three reactive sites and pentaerythritol with four available hydroxyl groups are the most commonly used alcohols. Methanol with a single hydroxyl group and tri-ethylene-glycol with two reactive sites are used to produce liquid and low softening point esters.
Figure 1: Alcohols Used to Produce Rosin Esters
Esterification is an equilibrium reaction that can be driven to near completion by removal of the water from the reactor, but there will always be some unreacted acid and residual alcohol in the final product. A typical acid number for a pure rosin acid is around 170. A glycerol ester typically has an acid value below 20. The type of alcohol chosen is key to determining the molecular weight of the rosin ester and its softening point. A typical softening point for glycerol esters is 85°C, and 105°C for pentaerythritol esters. The difference in softening point affects their compatibility and hence adhesive performance. Figure 2 shows a typical rosin esterification reaction with three molecules of rosin acid combining with glycerol to form a rosin tri-ester.
While the diverse array of rosin esters may seem bewildering at first, selection of a rosin ester for a particular application is driven by two considerations. The first factor is a determination of the color, odor, and stability requirements. If the application does not have critical requirements for these factors and low cost is the overriding concern then a rosin ester based on tall oil or gum rosin will be suitable. If low odor is very important, products based on gum rosin will be preferable because tall oil rosin carries some sulfur contaminants over from the sulphate Kraft paper process it is derived from. The second consideration is a balancing of softening point, tack, and viscosity factors. The compatibility of a rosin ester does not vary greatly between rosin sources or alcohol type. Switching from a glycerol ester to a pentaerythritol ester will generally increase cohesion and adhesion and will also increase the melt viscosity and softening point of the adhesive. Liquid rosin esters will increase tack and decrease viscosity and cohesion. If the base elastomer being used is low in polarity such as low vinyl acetate content EVA, polyethylene, or amorphous polyolefin, a hydrogenated rosin ester will be more compatible.
Figure 2: Esterification Reaction Producing a Typical Rosin Ester
For the ultimate in cohesion and high temperature resistance an ester of dimerized rosin resin is the preferred product. This produces a material with very high softening point.
In summary the selection of a rosin ester for a particular application depends on:
- The source of the rosin acid (gum, wood or tall oil rosin).
- The alcohol used to form the ester.
- The modification of the rosin prior to esterification (hydrogenation or dimerization).
With all of the process variables available to the formulator there is a rosin ester available in softening points from liquid to 178°C and colors from dark yellow to near white. For more information on rosin resins and a listing of products available in North America refer to Eastman publication WA-87, Eastman Rosin Products for Adhesives and Sealants.
Since transportation of some types of rosin resins is not feasible and all products are not produced at each manufacturing site, the availability of rosin resins varies regionally.